Gas sensor, in particular a lambda sensor

ABSTRACT

The reference air channel of a gas sensor or a lambda probe having a laminate body produced by printing technology is provided. The laminate body is produced by printing a suitably structured layer onto a neighboring layer, for example, by screen printing.

This application is a 371 of PCT/DE00/04472, filed on Dec. 14, 2000,which claims priority to German application 199-63-566.8, filed on Dec.29, 1999.

FIELD OF THE INVENTION

The present invention relates to a gas sensor, for example, a lambdaprobe, having a sintered ceramic laminate body, within which a referenceair channel is arranged inside a layer of the laminate. A electricresistance heater is situated (or embedded) in the laminate on one sidethereof, and an electrode arrangement is provided on the other side ofthe laminate, at least one reference electrode, which is gas-permeablein at least some areas, being arranged inside a bordering wall of thereference air channel, and a Nernst electrode, which is alsogas-permeable in at least some areas, being acted upon by a gas to besensed, in which the at least one reference electrode and the Nernstelectrode are separated by a solid electrolyte layer, which isconductive and permeable to ions, for example, oxygen ions.

BACKGROUND INFORMATION

Exhaust systems in modern internal combustion engines may be providedwith catalysts for catalytic decomposition of noxious exhaust gases, forexample, those occurring in motor vehicles. For good functioning of thecatalysts, air and fuel should be supplied to the engine in apredetermined ratio. For this purpose, engine controls are provided,which may be connected to a lambda probe. The signals of the lambdaprobe indicate the composition of the exhaust gases and thus permit theengine control to optimally regulate the ratio of fuel and combustionair for the catalyst.

In this regard, two concepts are described below.

The first concept attempts to achieve a stoichiometric combustion, i.e.,when the quantity of oxygen in the combustion air corresponds exactly tothe oxygen demand for complete combustion of the fuel supplied. In thiscase, the engine is not operated with an oxygen excess (λ>1) or anoxygen deficiency (λ<1). This type of operation may be characterized byλ=1.

For stoichiometric combustion, narrowband lambda probes may besufficient for exhaust gas sensing, the Nernst electrode being actedupon essentially directly by the exhaust gas.

This engine control considers the effect that an electric voltage, whichis generated by diffusion of oxygen ions and is detectable between thereference electrode and the Nernst electrode, changes in value in therange of λ=1. Thus, a signal that indicates a deviation from the desiredoperation of stoichiometric combustion is available. This signal mayindicate both a deficiency of oxygen as well as an excess of oxygen,with respect to stoichiometric combustion.

Such sensors are referred to in German Published Patent Application No.44 01 749, for example.

The second concept attempts to achieve operation of the combustionengine predominantly with an excess of oxygen in combustion, since thismay allow a significant reduction in fuel consumption. However, harmfulnitrogen oxides, which may form in combustion with excess oxygen, may beabsorbed by storage catalysts in the exhaust line of an automotiveengine only for a limited period of time. Before the storage capacity ofthese storage catalysts is depleted, operation of the engine must beswitched briefly to combustion with an oxygen deficiency, to permitreduction of the nitrogen oxides previously stored in the catalyst.These nitrogen oxides may accumulate, for example, due to incompletelyburned fuel constituents entering the exhaust gas line. The enginecontrol should therefore be switched repeatedly and intermittentlybetween a mode of operation that is predominant over a period of time inwhich the values of λ are above 1 and a relatively short-term mode ofoperation, in which the values of λ are less than 1.

Broadband lambda probes may be necessary for such intermittent operationwith greatly varying values of λ.

With such lambda probes, the Nernst electrode is arranged in a separatechamber, which communicates with the exhaust gas stream via a diffusionzone in the body of the probe. In addition, an internal pump electrode,which is situated inside this chamber, which may be connectedelectrically to the Nernst electrode, and which cooperates with anexternal pump electrode through a solid electrolyte layer, isessentially directly exposed to the exhaust gas stream. If an externalelectric voltage is applied between the two pump electrodes, both ofwhich may be gas-permeable in at least some areas, an oxygen ioniccurrent is generated between the pump electrodes in a directiondepending on the polarity of the applied voltage and with an amperagedepending on the electric potential difference. This external voltagepermits the control of the diffusion stream of the exhaust gases intothe diffusion chamber, for example, by a regulator which adjusts theexternal electric voltage between the pump electrodes and the electriccurrent occurring between the pump electrodes because of the oxygenionic current, so that an electric voltage having a predeterminedsetpoint is maintained between the reference electrode and the Nernstelectrode. Thus, the polarity and amperage of the electric currentbetween the pump electrodes may produce a signal in correlation with thecomposition of the exhaust gases and thus with the λ values.

Such probes are referred to in German Published Patent Application No.37 44 206, for example.

The probes described above should be heated during operation to generatea signal that may be analyzed. Therefore, lambda probes and other gassensors may have an electric resistance heater, which in the case of aprobe body formed by a laminate, may be situated on or between layers ofthe laminate.

SUMMARY OF THE INVENTION

In an exemplary embodiment according to the present invention, thereference air channel is situated in a structured layer or in a layerarrangement produced by printing technology.

It is believed that this offers the advantage in that various desiredshapes, including small parts, may be possible for the reference airchannel in comparison with a type of production, for example, in whichparts are punched out of a green ceramic film.

For example, the contours of the reference air channel may be adapted tothe contours of the electric resistance heater, which may meander, or ina top view of the layer planes they may be removed by an entrance holefor the exhaust gases passing through the probe body perpendicular tothe layer planes.

In addition, the reference air channel may be divided like a fan for theadmission of the reference air and/or to position the layer or layerarrangement forming the reference air channel on the longitudinal edges,the layer or layers of the layer arrangement optionally being brokendown into non-coherent parts, without thereby increasing themanufacturing complexity.

Due to the smaller cross sections of the reference air channel, whichmay be possible with production by printing technology, and due to thecomparatively small height of this channel perpendicular to the planesof the layer, a good heat conducting connection is created between theparts of the probe body on both sides of the layer or layer arrangementaccommodating the reference air channel, so that the thermal stressesoccurring at the start of heating within the probe body may remain lowand/or the probe body may be rapidly heated.

Moreover, the reference air channel may be filled with a porous mass,thus permitting a better heat transfer between the laminate parts onboth sides of the reference air channel.

Production of the reference air channel by printing technology may beaccomplished so that a negative pattern of the reference air channel isprinted on the side of a layer carrying the heater or sheaths theheater, this side facing the reference air channel, and/or on the side(of the solid electrolyte layer arranged between the reference electrodeand the Nernst electrode) carrying the reference electrode, this patternbeing printed using a pasty material, for example, zirconium oxidepaste, that hardens under heat.

In addition, the positive shape of the reference air channel may beprinted with a pasty material that partially or completely burns off,forming a porous structure. This may guarantee, or at least make moreprobable, for example, that the reference air channel has a reproducibleheight perpendicular to the planes of the layer of the laminate.

In summary, an exemplary embodiment according to the present inventionproduces the reference air channel by technical printing, toreproducibly produce various desired filigree structures with a lowmanufacturing complexity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view through a broadband lambda probe acrossline I—I in FIG. 2 in an area of the end of the probe body projectinginto the exhaust gas stream.

FIG. 2 is top view of the solid electrolyte layer between the referenceelectrode and the Nernst electrode as seen in a direction of arrow II inFIG. 1, showing the contours of the layer applied to the abovementionedlayer by printing technology for the reference air channel.

FIG. 3 is a cross sectional view of the end of the probe body on thereference air side parallel to the sectional plane of FIG. 1.

DETAILED DESCRIPTION

As shown in FIGS. 1 and 3, the lambda probe has a body 1 designed as aceramic laminate. The layers of the laminate are applied or stackedgreen. After subsequent sintering, which may occur after orsimultaneously with pressing of the laminate, a hard ceramic body 1 isproduced.

A bottom layer 2 of a thicker film of zirconium oxide is provided, abovewhich an electrically insulating double layer 3 is provided. An electricresistance heater 4 and respective printed conductors for the electricpower supply are embedded within the double layer 3, above which a layer5, which may be produced by screen printing, is structured. Layer 5 maybe made of, for example, zirconium oxide paste. Within this layer, areference air channel 6 is formed, an example of its outline beingillustrated in FIG. 2 and described below. This reference air channel 6has two end areas 6′ communicating with one another in the area of thesectional plane of FIG. 1.

Above layer 5 a solid electrolyte layer 7 is provided, for example, as azirconium oxide film, to which yttrium oxide has been added. At least inthe area of end areas 6′ of reference air channel 6, a gas-permeable,laminar reference electrode 8 made of a porous platinum material issituated on the side of layer 7 facing reference air channel 6 orbetween layers 5 and 7. This electrode 8 is connected to a terminalcontact on body 1, by an adjoining laminar printed conductor 8′, asdescribed below. (see FIG. 2).

Above solid electrolyte layer 8, a thin layer 9 is provided, which isstructured by printing technology and may be made of, for example,zirconium oxide paste. This layer 9 has a large recess, which isarranged centrally with respect to an exhaust gas admission hole 10passing through body 1 perpendicular to its layers. A porous material 12is deposited within this recess, leaving an annular space 11.

In the area of annular space 11, solid electrolyte layer 7 has agas-permeable laminar Nernst electrode 13 made of a porous platinummaterial.

Another solid electrolyte layer 14, for example, a zirconium oxide filmcontaining yttrium oxide, is provided above layer 9, i.e., porousmaterial 12. On its side facing annular space 11 and on its side facingaway from annular space 11, layer 14 has gas-permeable inner and outerpump electrodes 15 and 16 made of a platinum material that is porous inat least some areas. These electrodes 15 and 16 are shaped so that theyat least essentially cover annular space 11 as seen from the top of thelayers of body 1. A gas-permeable protective layer 17 is also providedabove layer 14.

For resistance heater 4 and various electrodes 8, 13, 15 and 16 to beelectrically accessible from the outside, contact lugs (not shown) aresituated on the reference air-side end of body 1. These lugs, which maybe produced by printing technology, are connected to resistance heater 4or electrodes 8, 13, 15 and 16 via through-contacts passing through oneor more layers and adjoining conductors running between adjacent layers.

As shown in FIG. 3, two through-contacts 18 passing through bottom layer2 are provided for connecting electric resistance heater 4. Thesethrough-contacts 18 may have, for example, an annular or cylindricalshape, as shown in FIG. 3.

In addition, a through-contact 19 designed in an annular or cylindricalshape and passing through the layers above layer 5 is provided forreference electrode 8. Contacts 18 and 19 may also be arrangedcoaxially.

To guarantee, or at least make more probable, reliable electricinsulation, despite the small thickness of layer 5 and the smalldistance between the facing ends of contact 19 and coaxial contact 18,the electrically insulating material of layer 5 is drawn intothrough-contact 19, so that the bottom end of through-contact 19, asshown in FIG. 3, is covered by electrically insulating material.

A through-contact 20 passing through layers 9, 14 and 17 is electricallyconnected to Nernst electrode 13 and to internal pump electrode 15.External pump electrode 16 is connected to a contact 21 via a printedconductor (not shown) passing through protective layer 17.

The production and structuring of layer 5 on layer 7 by printingtechnology are described below with reference to FIG. 2.

Reference electrode 8 and respective printed conductor 8′ are firstprinted on film layer 7, for example, by screen printing. Then, anegative image of reference air channel 6 and its end pieces 6′ and anyoptional fan-like mouths 6″ is applied with the material of layer 5.This may be performed, for example, by screen printing using appropriatemasks. It should be noted that extremely fine filigree structures may beoptionally produced in this manner.

If the coating of solid electrolyte layer 7 is performed with thematerial of layer 5, layer 7 is already stacked with the layers situatedabove layer 7, as shown in FIGS. 1 and 3. In addition, through-contact19 is also positioned. Accordingly, the material of layer 5 may be drawninto the interior at through-contact 19, so that this interior space iscovered by material 5′, at least in the lower area, as shown in FIG. 3,and thus is reliably insulated electrically at a later point in timewith respect to through-contact 18, which is coaxial withthrough-contact 19. This may guarantee, or at least make more probable,for example, that no conducting connection may be created betweencoaxial through-contacts 18 and 19 due to fouling.

A positive image of reference air channel 6 and its end pieces 6′ andmouths 6″ may optionally be printed onto layer 7 using a material, whichdissolves, burns off, or forms a porous, highly gas-permeable structurewhen body 1 is sintered.

Layer 3 may be printed with the material of layer 5 in mirror image tolayer 7. Also, optionally, layer 3 may be printed with the materialprovided for the positive image of reference air channel 6 and its parts6′ and 6″. In this manner, layer 5 may be produced with a greaterthickness.

The lambda probe described above functions as follows:

The end of body 1 having exhaust gas admission hole 10 is situated inthe exhaust gas stream, that is, in an area communicating with theexhaust gas stream of an internal combustion engine, while the other endof body 1 is acted upon by reference air, for example, atmospheric air.

Reference air reaches end pieces 6′ of the reference air channel throughreference air channel 6 via its mouths 6″. Exhaust gas passes throughexhaust gas admission hole 10 to porous material 12, through which theexhaust gas diffuses into annular space 11.

When body 1 is sufficiently heated by electric resistance heater 4, anelectric voltage may be detected between reference electrode 8 andNernst electrode 13 and thus between through-contacts 19 and 20. Thesize of this voltage depends on the oxygen partial pressures within endpieces 6′ of the reference air channel and within annular space 11.Specifically, the platinum material of aforementioned electrodes 8 and13 promotes or permits the formation of oxygen ions, resulting in an iondiffusion in solid electrolyte layer 7, which depends on theconcentration of oxygen ions on electrodes 8 and 13. This results in anelectric potential difference between electrodes 8 and 13.

The oxygen partial pressure in annular space 11 may be controlled byapplying an external electric voltage having a controllable polaritybetween pump electrodes 15 and 16. The corresponding voltage source isconnected to through-contact 20, i.e., to contact 21. In this manner, anoxygen ion current is produced having an amperage and directiondepending on the electric voltage and polarity of the external electricvoltage. This ion current diffuses through solid electrolyte layer 14,since the platinum material of electrodes 15 and 16 forms oxygen ions.Thus, an electric current is detectable between pump electrodes 15 and16.

A regulator may control the electric voltage and the electric currentbetween pump electrodes 15 and 16, so that the electric voltageavailable between reference electrode 8 and Nernst electrode 13corresponds to a fixed setpoint. The electric current available betweenpump electrodes 15 and 16 is a measure of the oxygen content of theexhaust gases relative to the reference air.

If external pump electrode 16 is at a positive electrical potential withrespect to internal pump electrode 15, the prevailing operatingconditions have λ>1. When the polarity is reversed, the prevailingoperating conditions have λ<1, the measure of the electric currentdetectable between electrodes 15 and 16 correlating with the size of λ.

The values of λ may be detected in a large value range.

In the case of narrow-band lambda probes, external protective layer 17may be above Nernst electrode 13, that is, layers 9 and 14, and pumpelectrodes 15 and 16 may be omitted. In this case, the electric voltagedetectable between electrodes 8 and 13 is a measure of the oxygenpartial pressure of the exhaust gases.

Depending on the design of the lambda probes for narrow-band orbroadband measurement, reference air channel 6, together with its parts6′ and 61″, may be produced in the manner described above by depositionof structured layer 5 by printing technology.

1. A gas sensor, comprising: a sintered ceramic laminate body including one of a structured layer and a structured layer arrangement, within which a reference air channel is arranged, the one of the structured layer and the structured layer arrangement being produced by a printing technology, and the reference air channel including a bordering wall; an electrically insulated electric resistance heater embedded in the laminate on one side of the laminate; an electrode arrangement arranged on another side of the laminate, the electrode arrangement including at least one reference electrode arranged inside the bordering wall of the reference air channel, the at least one reference electrode being gas permeable in at least selected portions, the electrode arrangement further including a Nernst electrode operable to be acted upon by a gas to be sensed, the Nernst electrode being gas permeable in at least selected portions; and a solid electrolyte layer separating the Nernst electrode from the at least one reference electrode, the solid electrolyte layer being conductive and permeable to ions, wherein the reference air channel includes mouths arranged in a fan like pattern, the mouths extending outwards towards at least one end of the one of the structured layer and the structured layer arrangement for admission of a reference air at the at least one end.
 2. The gas sensor according to claim 1, further comprising: through-contacts passing through at least one layer of the body and connecting contacts situated on an outside of the body to one of the following: (a) the electrodes of the electrode arrangement, and (b) printed conductors electrically connected to the electrodes of the electrode arrangement, wherein the one of the structured layer and the structured layer arrangement does not include any of the at least one layer through which the through-contacts pass.
 3. The gas sensor according to claim 1, wherein the solid electrolyte layer is conductive and permeable to oxygen ions.
 4. The gas sensor according to claim 1, wherein the printing technology includes screen printing.
 5. The gas sensor according to claim 1, wherein the reference air channel is filled with a porous material having gas permeable properties.
 6. A gas sensor, comprising: a sintered ceramic laminate body including one of a structured layer and a structured layer arrangement, within which a reference air channel is arranged, the one of the structured layer and the structured layer arrangement being produced by a printing technology, and the reference air channel including a bordering wall; an electrically insulated electric resistance heater embedded in the laminate on one side of the laminate; an electrode arrangement arranged on another side of the laminate, the electrode arrangement including at least one reference electrode arranged inside the bordering wall of the reference air channel, the at least one reference electrode being gas permeable in at least selected portions, the electrode arrangement further including a Nernst electrode operable to be acted upon by a gas to be sensed, the Nernst electrode being gas permeable in at least selected portions; a solid electrolyte layer separating the Nernst electrode from the at least one reference electrode, the solid electrolyte layer being conductive and permeable to ions; and through-contacts passing through at least one layer of the body and connecting contacts situated on an outside of the body to one of the following: (a) the electrodes of the electrode arrangement, and (b) printed conductors electrically connected to the electrodes of the electrode arrangement, wherein the one of the structured layer and the structured layer arrangement does not include any of the at least one layer through which the through-contacts pass, and wherein the through contacts have ring shaped cross sections provided with an electrically insulating coating on an inside of the through contacts.
 7. The gas sensor according to claim 6, wherein the gas sensor is a lambda probe.
 8. The gas sensor according to claim 6, wherein the solid electrolyte layer is conductive and permeable to oxygen ions.
 9. The gas sensor according to claim 6, wherein the printing technology includes screen printing.
 10. The gas sensor according to claim 6, wherein the reference air channel further includes a core that extends along a length of the at least one of the structured layer and the structured layer arrangement, and parts including at least one of (a) at least one reference air inlet and (b) at least one end piece, the parts extending from the core, and wherein, in comparison to a size of a surface area of the one of the structured layer and the structured layer arrangement used by the reference air channel, large surface areas of the one of the structured layer and the structured layer arrangement are provided at the electric resistance heater in a vicinity of one of the reference air channel and the parts of the reference air channel, the large surface areas being operable to heat conductively couple the resistance heater to the electrolyte layer separating the Nernst electrode from the at least one reference electrode.
 11. The gas sensor according to claim 6, wherein the reference air channel is filled with a porous material having gas permeable properties.
 12. The gas sensor according to claim 6, further comprising: an admission opening for receiving the gas to be sensed, the admission opening passing through at least one layer of the body and being perpendicular to a plane of the laminate and arranged within the body, wherein the one of the structured layer and the structured layer arrangement does not include any of the at least one layer through which the admission opening passes.
 13. The gas sensor according to claim 6, wherein the one of the layer and layer arrangement includes at least one short side and at least one long side, and the reference air channel opens outwardly towards at least one of the at least one long side of the one of the layer and layer arrangement including the reference air channel.
 14. The gas sensor according to claim 6, wherein the one of the structured layer and the structured layer arrangement including the reference air channel is made of the same material as the electrically insulating coating. 